RTP: send RTCP to clinet in server mode

Send RTCP correctly in the server mode to a client if we have the RTCP
connection (== receiving RTCP from the client).

It is done in the same way as it is currently implemented when UG is in
receiver-only mode without explicit remote address.

src/rtp/net_udp.c

@@ -905,11 +905,10 @@ error:
         return NULL;
 }
 
-static const struct in6_addr in6_blackhole = IN6ADDR_BLACKHOLE_INIT;
-
 bool udp_is_blackhole(socket_udp *s) {
         return s->sock.ss_family == AF_INET6 &&
-                        memcmp(&((struct sockaddr_in6 *) &s->sock)->sin6_addr, &in6_blackhole, IN6_BLACKHOLE_PREFIX_LEN) == 0;
+               is_addr_blackhole(
+                   &((struct sockaddr_in6 *) &s->sock)->sin6_addr);
 }
 
 void udp_set_receiver(socket_udp *s, struct sockaddr *sa, socklen_t len) {

src/rtp/rtp.c

@@ -1074,7 +1074,8 @@ struct rtp *rtp_init_if(const char *addr, const char *iface,
         session->opt = (options *) malloc(sizeof(options));
         session->userdata = userdata;
         session->mt_recv = multithreaded;
-        session->send_rtcp_to_origin = tx_port == 0 && is_host_loopback(addr);
+        session->send_rtcp_to_origin =
+            (tx_port == 0 && is_host_loopback(addr)) || is_host_blackhole(addr);
 
         if (rx_port == 0) {
                 const unsigned random_off = (ug_rand() % (IPPORT_MAX - IPPORT_DYNAMIC + 1)) & ~1U;

src/utils/net.c

@@ -3,7 +3,7 @@
  * @author Martin Pulec     <pulec@cesnet.cz>
  */
 /*
- * Copyright (c) 2016-2023 CESNET z.s.p.o.
+ * Copyright (c) 2016-2024 CESNET
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -41,6 +41,9 @@
 #include "config_win32.h"
 #endif
 
+#include <limits.h>
+#include <stdint.h>
+
 #ifdef WIN32
 #include <tchar.h>
 #else
@@ -156,6 +159,26 @@ static struct addrinfo *resolve_host(const char *hostname, const char *err_prefi
         return ai;
 }
 
+/// returns true for RFC 6666 IPv6 black hole address
+bool
+is_addr_blackhole(const struct in6_addr *addr)
+{
+        static const struct in6_addr in6_blackhole = IN6ADDR_BLACKHOLE_INIT;
+        return memcmp(addr, &in6_blackhole, IN6_BLACKHOLE_PREFIX_LEN) == 0;
+}
+
+/// returns true for RFC 6666 IPv6 black hole address string
+bool
+is_host_blackhole(const char *hostname)
+{
+        if (strchr(hostname, ':') == NULL) { // not an IPv6 literal
+                return false;
+        }
+        struct in6_addr ipv6_addr;
+        inet_pton(AF_INET6, hostname, &ipv6_addr);
+        return is_addr_blackhole(&ipv6_addr);
+}
+
 bool is_host_loopback(const char *hostname)
 {
         struct addrinfo *ai = resolve_host(hostname, "is_host_loopback: ");

src/utils/net.h

@@ -3,7 +3,7 @@
  * @author Martin Pulec     <pulec@cesnet.cz>
  */
 /*
- * Copyright (c) 2016-2021 CESNET z.s.p.o.
+ * Copyright (c) 2016-2024 CESNET
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -50,19 +50,22 @@
 #define IN6_MAX_ASCII_LEN 39 // 32 nibbles + 7 colons
 
 #define IN6ADDR_BLACKHOLE_INIT { { { 1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1 } } } // RFC 6666
-#define IN6_BLACKHOLE_PREFIX_LEN 8
+#define IN6_BLACKHOLE_PREFIX_LEN 8 // in bytes (64 bit prefix)
 #define IN6_BLACKHOLE_STR "100::1"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+struct in6_addr;
 struct sockaddr;
 struct sockaddr_storage;
+bool is_addr_blackhole(const struct in6_addr *addr);
 bool is_addr_linklocal(struct sockaddr *sa);
 bool is_addr_loopback(struct sockaddr *sa);
 bool is_addr_private(struct sockaddr *sa);
 bool is_addr_multicast(const char *addr);
+bool is_host_blackhole(const char *hostname);
 bool is_host_loopback(const char *hostname);
 bool is_host_private(const char *hostname);
 uint16_t socket_get_recv_port(int fd);